Arterial Anomalies 17 
appears at the cephalic end of the truncus and grows proximally 
to unite with the distal and proximal bulbar swellings and finally 
its interventricular border is attached to the interventricular sep- 
tum, while dorsally it probably assists in forming the pars mem- 
branacea septi. During this complicated process of division of 
the elongated truncus the aortico-pulmonary septum describes a 
spiral in a clock-wise direction of about 135 degrees. In lower 
forms (Lepidosiren) this spiral is carried for 270 degrees, Rob- 
ertson (1913), p. 195. Distally the septum extends laterally, di- 
viding the truncus into a dorsal (pulmonary) and ventral (aorta) 
compartment, fig. 18 I. Proximally the rotation of the septum 
through 135 degrees has changed the relations so that the dorsal 
compartment is the aorta and the ventral compartment is the pul- 
monary artery, fig. 18 Il. Through the rotation of the ventricu- 
lar cardiac loop which has been going on while this process is be- 
ing completed in the truncus, by the time the aorta and pulmonary 
artery have become independent vessels they have established 
definite relations with the left and right ventricles, fig. 18 III. 
We may consider this region as made up of four separate ele- 
ments, viz., truncus arteriosus, bulbus cordis, ventricular limb 
and arterial limb. 
The normal movements of rotation during development may be 
reversed, that is rotation may be counter clock-wise. When all 
the viscera share in this condition it is known as situs viscerum 
transversus. More than three hundred cases have been reported 
and an analysis of them shows that many are in every respect 
normal aside from the fact that the viscera and their arrange- 
ment is a mirror picture of the normal, Gruber (1865). What 
factors are operative in the production of situs viscerum trans- 
versus is not known. v. Baer described a chick which had ro- 
tated in the reverse direction and suggested that this might be 
the cause of the condition. Thomson (1830) accepted this 
theory, but it is wanting in proof. I have very carefully recon- 
structed the viscera of such an embryo which I have in my col- 
lection and find that in this specimen all structures are normal in 
position and development. Another theory which has been ad- 
vanced by several authors, Virchow (1861), is that situs trans- 
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